This invention relates to a hydrogen generation system and more particularly relates to a chemical hydride hydrogen generation system in combination with a fuel cell system.
Hydrogen has been recognized as an environmentally friendly clean fuel of the future since it has various applications in power generation systems. For example, hydrogen can be used as a fuel for fuel cells, especially proton exchange membrane fuel cells, which use hydrogen and air to produce electricity, generating only water as a by-product. Fuel cells are being developed to replace traditional electricity generators because they produce clean, environmentally friendly energy. However, these fuel cells require external supply and storage devices for hydrogen. Extensive efforts have been made to develop a safe and efficient way to store hydrogen, especially in mobile applications. Conventional hydrogen storage technologies include liquid hydrogen, compressed gas cylinders, dehydrogenation of compounds, chemical adsorption into metal alloys and chemical storage as hydrides. However, each of these systems is either hazardous or bulky.
Another method of storing hydrogen has been proposed recently. This method uses a classical chemical hydride, such as NaBH4, as a hydrogen storage medium. The principle of this method is the reaction of the chemical hydride with water in the presence of a catalyst to generate hydrogen, as shown in the equation below:
NaBH4+2H2Oxe2x86x924H2+NaBO2+HEAT 
The borohydride, NaBH4, acts as both the hydrogen carrier and the storage medium. Ruthenium, Cobalt, Platinum or alloys thereof can be used as a catalyst in this reaction. It is to be noted that this reaction occurs without a catalyst in an acidic environment and only slightly under alkali conditions. This means the chemical hydride solution can be stored and has a shelf life under alkali conditions. This reaction is efficient on a weight basis since half of the hydrogen produced comes from NaBH4 and the other half comes from H2O. Borohydride is a relatively cheap material, usually used in wastewater processing, pharmaceutical synthesis, etc. Borohydride is also easier and safer to handle and transport than highly pressurized hydrogen or cryogenic hydrogen. As a result, there are some advantages to use borohydride as a method of storing hydrogen as a fuel for fuel cells.
There are several known examples of hydrogen generation systems that utilize chemical hydrides. One type of hydrogen generation system comprises a closed vessel for containing chemical hydride and a mechanical stirring mechanism disposed within the vessel for stirring the chemical hydride within the vessel. Water is injected into the vessel to react with chemical hydride and generated hydrogen is removed from the vessel through an outlet. The stirring mechanism means is used to ensure sufficient contact between the hydride and water while preventing the clumping of the hydride. Since the hydride is in solid phase in this system, the stirring mechanism is indispensable. However, in such systems the stirring mechanism consumes energy, increases the overall system weight and reduces system efficiency. Further, the noise generated in the stirring operation is undesirable. In addition, the reaction rate is low, making the fuel unresponsive, useless or very hard to control. The system also tends to be large and cumbersome.
Another type of hydrogen generation system employs a chemical hydride solution. In this system an aqueous chemical hydride solution is introduced to a catalyst bed to generate hydrogen. However, there are a number of problems associated with this liquid phased system. First, the by-product borate, in the above equation, NaBO2 is less soluble then the reactant borohydride, namely NaBH4. Specifically, NaBO2 is only approximately 20% soluble. This means that in order to generate hydrogen in a liquid phased system, and thereby reduce the problems associated with the aforementioned solid phased systems, the concentration of borohydride in the solution can only be about 20% which is much lower than borohydride""s solubility in water. Therefore the achievable hydrogen density of the system is considerably limited.
A further deficiency of the aforementioned examples is that neither system is capable of responding in real time to the fuel (hydrogen) needs of the fuel cell. This ability is referred to as load following ability.
In order to overcome the aforementioned deficiencies associated with the prior art, one aspect of the present invention provides an energy system comprising:
a fuel cell stack for generating electricity from hydrogen and an oxidant to form water;
a chemical hydride hydrogen generation system, comprising:
a storage means for storing a chemical hydride solution comprising a solution of a chemical hydride solute in a solvent; a reactor containing a catalyst, for catalyzing reaction of the chemical hydride to generate hydrogen; a first pump means connected between the storage means and the reactor in a first circuit, for circulating the chemical hydride solution through the storage means and the reactor, so that the chemical hydride reacts to generate hydrogen in the presence of the catalyst;
a first connection between the chemical hydride generation system and the fuel cell stack for supplying hydrogen to the fuel cell stack; and
a heat transfer circuit including second connections between the chemical hydride generation system and the fuel cell stack, for circulation of the chemical hydride solution through the fuel cell stack to effect heat transfer between the fuel cell stack and the chemical hydride solution
The chemical hydride solution can be a borohydride hydride water solution. The solute of the solution can be in the form of MBxHy, in which M is a metal. Specifically, the solute can be NaBH4, LiBH4, KBH4, RbH4. Alternatively, the solute can be NH3BH3. Preferably, the chemical hydride solution is a water solution in which the solute is NaBH4 and less than 5% LiBH4. Preferably, to ensure the system works properly under low temperature, the chemical hydride solution further includes a freezing point depressing agent. The freezing point depressing agent is preferably glycerol and concentration of glycerol is no higher than 5%. More preferably, the concentration of glycerol is 1%. The solution preferably further includes alkaline additives. More preferably, the alkaline additive is selected from LiOH, KOH, and NaOH. More preferably, the alkaline additive is 0.1% NaOH.
More preferably, the system further includes a flow control means that operatively stops the first pump means when the hydrogen pressure in the reactor reaches a first value and activates the first pump means when the hydrogen pressure in the reactor reaches a second value lower than the first value. More preferably, the system further includes a heat exchanging means for the reactor that selectively removes heat from the reactor during normal operation and heats up the reactor when the system works under low temperature.
According to another aspect of the present invention, there is provided a method of operating a fuel cell stack and chemical hydride hydrogen generation system with a reactor including a catalyst for generating hydrogen for the fuel cell and controlling the temperature of the fuel cell stack and the chemical hydride generation system, the method comprising the steps of:
1) supplying a chemical hydride solution to the reactor, and permitting the catalyst to catalyze reaction of the chemical hydride solution to generate hydrogen;
2) supplying the hydrogen to the fuel cell stack and supplying an oxidant to the fuel cell stack, for generation of electricity;
3) circulating the chemical hydride solution through the fuel cell stack and the chemical hydride generation system, to effectively transfer heat therebetween.
Preferably, the means for recovering the water generated in the fuel cell includes a gas-liquid separator. More preferably, the system further includes a switch means that selectively allows the excess hydrogen leaving the fuel cell after reaction to be circulated back to the fuel cell, in the first mode and allows the hydrogen to be supplied to the catalytic burner from the fuel cell in the second mode. More preferably, the system further includes a first control means that operatively switches the switch means between the first and second modes.
Preferably, the means for supplying hydrogen generated in the reactor to the fuel cell further includes a filtering means between the reactor and the fuel cell for purifying the hydrogen generated in the reactor before the hydrogen is supplied to the fuel cell.
In order to provide the energy system with load following capability, the system further includes a second control means that operatively stops the first pump means when the hydrogen pressure in the reactor reaches a first value and activates the first pump means when the hydrogen pressure in the reactor reaches a second value lower than the first value.
In order to ensure that the energy system works properly under low temperature, the system further includes a heat exchanging means for the reactor that selectively removes heat from the reactor during normal operation and heats up the reactor when the system works under low temperature.
The fuel cell stack can comprise a single fuel cell or can include a plurality of fuel cells, and coolant ducts can be provided for the or each fuel cell.
The chemical hydride hydrogen generation system according to the present invention provides a safe, clean, efficient and reliable hydrogen generation system and an energy system in which the hydrogen generation system and the fuel cell system operate synergistically. The hydrogen generation system is safe in that low pressure hydrogen is generated and used in the fuel cell instead of highly pressurized hydrogen. The system is also environmentally safe in that the reaction products are harmless detergent base chemicals. When novel borohydride solution is used, the system can operate at as low as xe2x88x9222xc2x0 C. temperature. The pressure control means employed in the system enables the system to follow the load of fuel cell stack as well as meet peak performance requirements. By capturing and recycling the water in the fuel cell exhaust and introducing it into the hydride solution, the system of the present invention further enhances the energy density. Experiments show that the chemical hydride hydrogen generation system according to the present invention has achieved energy densities of 1.2 KWh/L and 0.8 KWh/kg, which is comparable, if not advantageous to fuel cell systems currently available. Hydrogen recycled through a filtration system could also allow for higher system efficiency as well as an increased chemical hydride energy density. The circulation of the chemical hydride solution as a heat transfer fluid between the chemical hydride reactor and the fuel cell stack avoids the need for a separate heat transfer fluid and provides the advantage of mutually warming up the chemical hydride reactor and the fuel cell stack, during initial start up, thereby shortening the time needed for the system to achieve optimum operation condition, and further improving the system efficiency.